The demand for improving the capacity, speed and quality of wireless data services is driving advancements in the spectral efficiency and error correction capability of wireless communication systems.
Improvements in spectral efficiency may be realized using multi-carrier modulation techniques which can use a number of closely spaced, non-interfering orthogonal sub-carriers that overlap in frequency. Such modulation techniques include Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA), which have been successfully used because of their efficiency and high tolerance to adverse multipath effects. However, these modulation techniques may produce transmit signals having a peak power level which is much larger than the average power of the transmitted signal, thus having a high Peak-to-Average-Power Ratio (PAPR). Signals having large peaks may increase the amount of intermodulation distortion resulting in an increase in the error rate. Minimizing the PAPR allows a higher average power to be transmitted for a fixed peak power, which may improve the overall signal-to-noise ratio at the receiver.
The PAPR of a multi-carrier modulated signal may be managed using a variety of approaches which introduce some redundancy into the communication system. One known PAPR management technique is called Selective Mapping (SM), which may involve generating a large set of data vectors all representing the same information. The data vector with the lowest resulting PAPR may be selected for transmission. Another PAPR management approach is called Partial Transmit Sequences (PTS), which rotates the phase of data vectors creating a set of candidate signals conveying similar information. The rotated phase signal providing the lowest PAPR is chosen for transmission.
As mentioned above, the increasing demand for improved wireless communication systems are also driving improvements in error correction techniques. Transmission errors may be attributable to competing signals being simultaneously transmitted (especially in the case where signals have an undesirably high PAPR), inclement weather, signal strength shadows, electrical interference and/or other conditions affecting the air interface. Such conditions may result in one or more packets being dropped or corrupted. When a dropped or corrupted packet occurs, additional resources may be utilized to provide redundant information to correct the transmission error. One method of providing additional information is to selectively increase redundancy using HARQ processing. HARQ processing can enable error recovery by combining retransmitted packets with previously received corrupted packets stored at the receiver.
While HARQ processing may be effective at correcting errors, it can introduce undesirable latencies into a communication system. HARQ retransmissions conventionally involve redundant encoding operations, which may be computationally intensive and thus consume time and power.
Accordingly, it would be desirable to improve the efficiency of error correction for digital communications by reducing HARQ retransmissions during error correction operations.